This invention relates to vehicle wheel slip control and more particularly to control of wheel slip for electric motor-driven vehicles such as diesel locomotives.
Diesel-powered railroad road locomotives generally utilize an electrical power transmission system involving a diesel engine-driven generator connected with a plurality of series wound traction motors each driving a powered axle supported by and driving a pair of rail-engaging wheels. A current system utilizes DC traction motors with a control system that constantly seeks to maximize the power delivered to the traction motors, within the power capability of the engine, by adjusting the field current of the main generator. In cases where wheel to rail adhesion is marginal, a wheel set may begin to slip. A wheel slip control detects this slip and promptly reduces main generator excitation, reducing output power to stop the slip, after which power is reapplied. A result of this method of operation is that power is reduced to all the traction motors powered by the generator even though only one wheel set may be slipping.
By contrast, locomotives may be provided with AC traction motors that operate with several induction motors fed from a variable frequency, variable voltage inverter. This control system has two advantages over the DC traction motor system previously described. First, in the event of a wheel slip, the motor speed can increase only a few percent until the rotation frequency of the motor approaches the inverter supply frequency. Prevention of a large speed increase during wheel slip is inherent, requiring no constant monitoring of wheel speeds as in the DC traction motor system. Also, the power supply to a slipping motor is inherently reduced by a much greater amount per unit of wheel slip than in the DC traction motor system. Second, during wheel slip, the power to non-slipping motors does not have to be reduced. They can continue to operate normally. Because of these advantages, AC traction motor locomotives can reliably operate at higher percent adhesion, that is, closer to the point of wheel slip, than DC traction motor locomotives as described above.
There are, however, other control schemes whereby DC traction motor locomotives can operate at higher percent adhesion comparable to, and perhaps even better than, AC traction motor locomotives. One of these uses separately excited fields for the DC traction, motor locomotives. Separate excitation has been used on previous electric locomotives and was able to limit slipping wheel speed to a few percent over nominal speed and power to non-slipping motors was not reduced, much the same as with the AC traction motor locomotives.
However, separate excitation for each traction motor adds considerably more expense compared to common excitation. A better and more cost effective method of wheel slip control is accordingly desired.
The present invention provides an alternative wheel slip control for DC traction motor locomotives having series excitation. The alternative control involves motor field shunting, also known as field weakening, for providing wheel slip control. With this arrangement, each traction motor has its field shunted (bypassed through a parallel resistor) by some small percentage, such as 10%, during normal motoring operation. In the event that slipping of a wheel set is sensed by the control system, the motor powering the slipping wheel set is controlled by opening the shunting circuit, increasing the field current, and causing the motor""s armature current and power to be dramatically reduced, thus stopping the wheel slip and allowing shunting of the motor field to be reapplied.
In the past, field shunting has sometimes been applied to all the traction motors connected to a generator in order to allow the locomotive to be operated at higher speeds without exceeding the voltage limits of the generator or traction motors. However, it is not believed that removal of field shunting has previously been applied for controlling wheel slip in locomotives, and particularly for controlling slipping of individual traction motor wheel sets without affecting the power delivered to other wheel sets in the locomotive.